1. Field of the Invention
The present invention relates to the field of temperature stabilization. Particularly, the present invention is related to the field of temperature stabilized ovens used to isolate crystal oscillators from variations in temperature of the external environment.
2. Description of the Prior Art
Crystal ovens, such as the one described in U.S. Pat. No. 3,040,158 issued to Leonard S. Cutler, Palo Alto, California, on June 19, 1962, typically consist of a quartz crystal mounted on a thermally conductive base which is thermally isolated from the external environment. The temperature of the thermally conductive base is sensed at a location close to the crystal by a temperature sensor, such as a thermistor. The thermistor is coupled to an oven controller which controls the power applied to a heater, also mounted to the thermally conductive base. In a proportional oven, the oven controller varies the current to the heater or the duty cycle of the heater voltage in response to the difference between the sensed oven temperature and the desired oven temperature. In this type of oven, the temperature of the crystal is relatively insensitive to variations in the external temperature.
The term "thermal gain" is a figure of merit which describes the temperature stabilization of a crystal oven. More specifically, thermal gain is defined as the ratio of the change in the external temperature to the corresponding change in temperature of a reference, such as the quartz crystal or the temperature sensor. As described above, the thermal gain at the precise location of the temperature sensor can be made quite high by using high gains in the negative feedback control loop of the oven controller. However, it has been empirically determined that the thermal gain between the crystal and the external environment is lower than the thermal gain between the temperature sensor and the external environment even when the sensor and the crystal are mounted very close to one another. Further, it has been determined that different locations on the thermally conductive base are characterized by significantly different thermal gains. Therefore, in order to obtain the desired high thermal gain between the crystal and the external environment, it has become common practice to physically move either the crystal, the sensor, or the heater until a high thermal gain is obtained.
It has also been determined that the thermal gain of specific locations on the thermally conductive base vary as the thermal geometry of the thermally conductive base is changed. This results in a change in the thermal gain of a specific location of the thermally conductive base every time a small hole is drilled in the base, part of the base is milled, or a new component is attached to the base. Thus, a change in the thermal geometry of an oven may result in a need to relocate the crystal or a control element.
In practice, the need to relocate elements by trial and error in order to achieve high thermal gains between the crystal and the external environment creates great difficulties in the design of a crystal oven. This is especially so since this trial and error relocation must be redone every time the thermal geometry of the thermally conductive base is changed.